Thermal ink jet printers are well known in the prior art as exemplified by U.S. Pat. No. Re. 32,572 issued to Hawkins et al. In the system disclosed in this patent, a thermal printhead comprises one or more ink-filled channels communicating with a relatively small ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. A plurality of heating resistors are located in the channels at a predetermined distance from the nozzle. The heating resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. Typically, the ink is water-based, and the bubble that forms consists of water vapor. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
In the channels, the heating resistors are subject to wear from corrosive ink as well as from mechanical shock produced by collapsing bubbles and thermal fatigue. In particular, the temperature of the ink adjacent an active heating resistor reaches at least 300 degrees centigrade, which is the temperature at which bubble nucleation occurs. Since the expected lifetime for commercial heating resistors is at least 200 million firings, measures are taken to protect the heating resistors. One measure is to construct the heating resistors to withstand the wear. For example, U.S. Pat. No. 4,931,813 to Pan et al. discloses forming the heating resistor from a relatively thick layer of unpassivated resistive material, such as TaAl. While this approach is generally adequate, it has the disadvantage that direct exposure of the heating resistors to the ink and cavitation forces can cause wear of and changes to the heating resistors. These effects can result in nonuniform print quality.
Another measure is to cover the heating resistors with protective layers, thus sparing the resistors from direct contact with the ink. For example, U.S. Pat. No. Re. 32,572 issued to Hawkins et al, U.S. Pat. No. 4,774,530 to Hawkins and U.S. Pat. No. 4,935,752 to Hawkins disclose covering heating resistors and associated electrodes with a passivation layer of silicon dioxide, silicon nitride, or both. In addition, a tantalum layer may be deposited on the passivation layer above the heating resistors for additional protection against cavitation forces. Similarly, U.S. Pat. No. 4,951,063 to Hawkins et al. discloses covering heating resistors with a high temperature deposited plasma or pyrolytic silicon nitride layer followed by a tantalum layer. Tantalum layers are strong and resist corrosion.
While the tantalum layer generally provides adequate protection, it is subject to erosion. One mechanism for erosion is hydrogen embrittlement, a process whereby a metal, such as tantalum, absorbs hydrogen and becomes brittle. Brittle tantalum can be easily fractured, particularly since the tantalum layer is subject to cavitation forces when a bubble collapses. Hydrogen can be absorbed into many materials if a voltage bias is present. Moreover, even without a bias voltage, tantalum can absorb hydrogen if the temperature of the tantalum is sufficiently high. For example, absorption occurs without bias at the operating temperature of a typical thermal ink jet. In a typical thermal ink jet, the temperature on the tantalum layer surface reaches at least 300 degrees centigrade, the temperature at which bubble nucleation occurs. After nucleation, the temperature exceeds the nucleation temperature because the heating resistor is still producing heat and the newly formed bubble insulates the heating resistor from the heat-conducting ink. The temperature can reach 450 degrees centigrade.
The source of the hydrogen is the hydronium ion (the hydrated proton, H.sub.3 O.sup.+). The hydronium ion is always present in the water in the water-based ink. Aside from hydronium ions normally present in water, the ink typically contains a greater concentration of hydronium ions because it is salted and acidic. The ink is salted to make it conductive to aid in sensing the amount, or absence, of ink in a printhead. Moreover, the ink is made acidic to avoid the etching of tantalum and of silicon that results from alkaline water.
Another mechanism for erosion of the tantalum layer is electrochemical reaction between the tantalum and the ink. The reaction is increased by voltage transients or spikes that pass through the tantalum layer during the rise and fall of a current pulse through the heating resistor associated with that particular tantalum layer. The voltage spikes are caused by capacitive coupling between the tantalum layer and its heating resistor. Capacitive coupling occurs because the tantalum region is separated from the heating resistor by an insulating dielectric layer, forming a capacitor between the tantalum layer and its heating resistor.
Significant capacitive coupling occurs unless the RC time constant of the tantalum layer and surrounding environment is much less than the rise and fall times of the current pulses. Typically, the current pulses have a period of 5 microseconds, and correspondingly short rise and fall times (e.g., 10 to 50 nanoseconds). The rise and fall times are particularly quick for printheads having the current pulse driver transistors located on the same integrated circuit substrate as the heating resistors. (Placing drive transistors and resistors on the same substrate is popular because it allows multiplex addressing of the drive transistors, which reduces the number of leads connected to the substrate. Placing drive transistors on the same substrate, however, reduces the capacitive load to the driver transistors, which also decreases the rise and fall times.) For calculating the RC time constant, typically there is a capacitance of about 3 picofarads between a tantalum layer and its associated resistor. The resistive component of the RC time constant is mainly the resistance from the tantalum layer to ground through the conductive ink contacting the tantalum layer. The ink resistance depends largely on the salt content of the ink. Ink resistances range from 1000 ohms to 50,000 ohms, with 10,000 ohms being a typical value. For the typical ink resistance of 10,000 ohms, the RC time constant is about 30 nanoseconds. For this case the magnitude of the voltage spikes approaches its theoretical maximum of half the voltage across the heating resistor.